1. Field of the Invention
The present invention relates to a method of improving performance of thin-film semiconductor devices having pairs of electrodes separated by a semiconductor film and containing electrical shorts or shunts. More particularly, the present invention relates to a method of electrochemically removing the effect of electrical shorts and shunts contained in large-area photovoltaic modules.
2. Description of the Related Art
As is well known in the art, the performance of thin-film amorphous silicon photovoltaic devices is adversely affected as the active surface of the device is increased. This phenomonon is primarily due to the creation of electrical shorts and shunts during fabrication of large-area photovoltaic devices.
In a conventional p-i-n or n-i-p amorphous silicon photovoltaic module comprised of a plurality of photovoltaic cells, each cell is comprised of a thin-film active semiconductor body, typically including a plurality of layers of semiconductor material, sandwiched between two conducting electrodes. Electrical shorts occur when the two electrodes come into electrical contact through a conductive metal path extending through the semiconductor body. This path can be caused by a local point defect, which either prevents the formation of the semiconductor layers during manufacture of the device, which typically is by a glow discharge deposition process, or causes the semiconductor layers to be peeled off. An electrical shunt is the loss of charge in the semiconductor body due either to an imperfect rectifying barrier or to the formation of an ohmic contact via a high work-function metal.
A conventional multi-cell hydrogenated amorphous silicon (a-Si:H) photovoltaic module of the type used for recharging electrical storage batteries is shown schematically in FIG. 1 and designated generally by reference numeral 6. The module 6 of FIG. 1 comprises n series-connected photovoltaic cells 8 formed on a substrate 10. Each cell 8 comprises a front electrode 12 formed of a conductive transparent oxide (CTO), a thin-film semiconductor body 14 typically formed of a-Si:H and its alloys in a p-i-n structure, and a metallic back electrode 16. As shown in FIG. 1, the ith cell 8(i) is comprised of front electrode 12(i), thin-film semiconductor body 14(i), and back electrode 16(i). Grooves formed between adjacent front electrodes 12(i) and 12(i+1) are filled with the amorphous silicon material forming overlying semiconductor body 14(i) to electrically insulate the adjacent front electrodes from each other. Adjacent semiconductor bodies 14(i) and 14(i+1) also are separated by a groove 13 to expose the underlying front electrode 12(i+1) and permit the formation of the series connections between adjacent cells.
With reference to FIG. 2, which shows a portion of module 6 of FIG. 1 during an intermediate stage of fabrication, the back electrodes 16 and the series connections between adjacent cells typically are formed by first depositing a layer 16' of a metal such as aluminum to cover the photovoltaic bodies 14(1) through 14(n). Then, the portions of metal layer 16' within width dimension w at each cell are removed by, for example, chemical etching or laser scribing to separate adjacent back electrodes 16 while retaining the connection between each metal back electrode 16(i) and the adjacent CTO front electrode 12(i+1), as shown in FIG. 1. The resulting back electrodes 16 are approximately L-shaped in cross section, each having one portion 16a of thickness t.sub.1 overlying semiconductor body 14 and a second interconnection portion 16b of thickness t.sub.2 that contacts the adjacent CTO front electrode. Thickness t.sub.1 of portion 16a normally is greater than thickness t.sub.2 of portion 16b.
The point defects that typically cause shorts and shunts in conventional devices are shown schematically in FIG. 1 and are designated by reference numeral 20. The density of these point defects has been found to increase rapidly as the surface area of the device is increased. Although the origin of these defects is not completely understood, they might be caused during the deposition of the CTO electrodes, by improper cleaning of the CTO electrodes prior to the deposition of the amorphous silicon semiconductor bodies, or due to dust generation during the deposition of amorphous silicon. Whatever the actual cause of these point defects, they lead to a reduction in the output and efficiency of large-area photovoltaic modules.
Attempts have been made in the art to remove electrical shorts and shunts from a thin-film photovoltaic device by applying a reverse-bias voltage to the device. A reverse-bias voltage is defined as an applied voltage having a polarity opposite that of the photovoltage generated by the device when exposed to light. When free of defects, the semiconductor material in the photovoltaic device ideally will act as a diode and prevent current flow from being induced by the reverse-bias voltage. (In actuality, a nominal leakage current called "reverse saturation current" will be induced by a reverse-bias voltage even in a "defect-free" cell.) When point defects exist, the leakage current flows at relatively low resistance through these defects. If the reverse-bias voltage is sufficiently high, the leakage current will be large enough to burn out or oxidize the electrodes at the point defects, thus effectively rendering the defects non-conductive and "curing" the device. The usefulness of such dry reverse-bias electrical curing is limited, however, because the high reverse-bias voltage required to oxidize the electrodes at the defects often produces new shorts and shunts.
Several methods of electrochemical curing using reverse-bias voltage also have been tried in the past. For example, U.S. Pat. No. 4,543,171 to Firester et al. discloses a method wherein a photovoltaic device having exposed aluminum electrodes is immersed in a liquid or gaseous bath of a chemical etchant consisting of an aluminum-etching acid mixture diluted to one part in three parts water or a gaseous composition of, for example, hydrochloric acid or flourine. The etchant has an etching rate that increases with increased temperature so that, when a reverse-bias voltage is applied to the photovoltaic device immersed in the etchant bath, localized heating of the exposed electrodes occurs at the shunts and shorts, where the leakage current flows at a relatively high current density. The localized heating increases the etching rate of the etchant at the shorts and shunts and causes the etchant to etch away or oxidize the exposed electrodes at the defects, thereby rendering the defects non-conductive.
Certain precautions must be made, however, to prevent the method disclosed by Firester et al. from damaging rather than curing the device. For example, Firester et al. teach that it is important to have a thin barrier layer of titanium between the semiconductor body and the exposed aluminum (or silver) electrode to prevent damage to the immersed device while the reverse-bias voltage is applied. Furthermore, Firester et al. teach forming a passivation layer of SiO.sub.2 or an organic resin over the exposed electrode to mask portions of the exposed electrode from etching. This passivation layer must be removed before the module can be used, hence increasing the number of manufacturing process steps required.
We have found that, when the method disclosed by Firester et al. is applied to a conventional large-surface-area multi-cell photovoltaic module such as that shown in FIG. 1 lacking protective barrier and passivation layers, the photovoltaic module typically is damaged by chemical etching at locations remote from the shunts and shorts, particularly at the interconnections between adjacent cells. Although the precise cause of the damage is uncertain, we believe that two plausible explanations exist. First, the electrical connection at the interface between a metallic back electrode 16(i) and the adjacent CTO front electrode 12(i+1) might not be a perfect ohmic connection so that a local temperature increase results at the interface during the application of the reverse-bias voltage. As a consequence, the etching rate of the etching solution would be increased at the interface with a resulting unintended removal of electrode material. Second, in the absence of a passivation layer, the liquid etching ambient disclosed by Firester et al. will etch the aluminum of back electrodes 16 at the interconnection portion 16b, where the material has the small thickness t.sub.2, even at room temperature. Consequently, the longer the photovoltaic module is immersed in the etching ambient, the greater the likelihood that etching at undesirable locations will take place.
The present invention is directed to providing an improved method of electrochemically etching electrical shorts and shunts to effectively remove them from a thin-film semiconductor device without causing damage to the interconnections between adjacent cells.
The present invention also is directed to providing a method of effectively removing electrical shorts and shunts from a thinfilm semiconductor device that does not require an elaborate etchant bath chamber.
Additional advantages of the present invention will be set forth in part in the description that follows and in part will be obvious from that description or can be learned by practice of the invention. The advantages of the invention can be realized and obtained by the method particularly pointed out in the appended claims.